The present invention is directed to a process for producing a molded article, and more particularly to a process for producing a molded article, capable of producing a molded article having a shape accurately corresponding to the shape of an inner surface of a mold, and a surface-treating agent used in the process.
When a molded article is produced by pouring or injecting a liquid molding material into a mold, and curing the liquid molding material, a mold releasing agent has been hitherto applied to the inner surface of the mold in order to easily release a molded article from the mold.
However, when a mold releasing agent is applied to the inner surface of a mold having a complicated shape on its inner surface, and a molded article is produced, the surface properties of the molded article are sometimes deteriorated because so-called surface void, i.e. surface defect caused by insufficient filling of the liquid starting materials and/or foams into the mold having a complicated shape on its inner surface is generated.
An object of the present invention is to provide a process for producing a molded article, capable of producing a molded article having a shape accurately corresponding to the shape of an inner surface of a mold, without the generation of defects which lower the surface properties of a molded article, such as surface void on the surface of the molded article.
The above and other objects of the present invention will be apparent from the following description.
The present invention is directed to the following:
[1] a process for producing a molded article comprising,
(A) pouring or injecting a liquid molding material into a mold in the presence of a surface-treating agent comprising a contact angle-reducing substance under the condition that the contact angle between the liquid molding material and the inner surface of the mold is not more than 30xc2x0, and
(B) curing the liquid molding material;
[2] a surface-treating agent used in a process for producing a molded article comprising pouring or injecting a liquid molding material into a mold and curing the liquid molding material, comprising a contact angle-reducing substance which reduces the contact angle between the liquid molding material and a flat plate made of the same material as the mold to not more than 30xc2x0, when the liquid molding material containing the surface-treating agent is dropped on the flat plate, or when the surface-treating agent is applied to the flat plate and the liquid molding material is dropped thereon;
[3] a method for reducing surface voids during molding, comprising using the above surface-treating agent; and
[4] use of the above surface-treating agent for reducing surface voids.
Typical examples of the liquid molding materials are resins such as polyurethanes, epoxy resins, phenolic resins, polyesters, urea resins, olefinic resins such as polyethylenes and polypropylenes, and styrenic resins; rubbers such as natural rubbers, isoprene rubbers, chloroprene rubbers, styrene-butadiene rubbers, butadiene rubbers, acrylonitrile-butadiene rubbers, ethylene-propylene rubbers, butyl rubbers, and acrylic rubbers.
The liquid molding materials can be those which are foamed during molding to give a foamed molded article. The liquid molding materials which give a foamed molded article include self-foamable polyurethanes, foamable olefinic resins, foamable styrene resins, or the like. When the olefinic resins, the styrene resins, or the like are used, there can be employed a process comprising foaming pre-foamed resin particles, or a process comprising impregnating a foaming agent into those resins, and thereafter molding and foaming the resins in a mold.
The process of the present invention exhibits excellent effects for liquid molding materials, particularly polyurethanes, the surface properties of which are easily deteriorated during molding. Typical examples of the polyurethanes are, for instance, polyether polyurethanes, polyester polyurethanes, and the like.
The starting materials for the polyurethane are not particularly limited, and known ones can be used. It is desired that the starting materials for the polyurethane are a polyol solution and an isocyanate prepolymer. The polyol solution comprises a polyol component, such as a polyether-polyol or a polyester-polyol, a chain extender, water, a foam stabilizer (a surfactant), and, as occasion demands, a catalyst. The isocyanate prepolymer can be prepared from a polyol component such as a polyether-polyol or a polyester-polyol, and a polyisocyanate component such as methylenediphenyl diisocyanate or a modified compound thereof. The polyether-polyol, the polyester-polyol, the chain extender, the foam stabilizer (a surfactant), the catalyst, the polyisocyanate component, and the isocyanate prepolymer may be those which are known. The contact angle-reducing substance can be contained in the isocyanate prepolymer and/or the polyol solution, and it is desired that the contact angle-reducing substance is contained in the isocyanate prepolymer. The content of the contact angle-reducing substance in the isocyanate prepolymer is preferably 0.1 to 7% by weight, more preferably 0.1 to 5% by weight.
The polyurethane foam containing the contact angle-reducing substance can be produced by reacting the polyol component with components such as the polyisocyanate component, the isocyanate prepolymer, and as occasion demands, water, a chain extender, a foam stabilizer (a surfactant), a catalyst, and the like, in the presence of the contact angle-reducing substance.
In the present invention, one of the major features resides in that the liquid molding material is poured or injected into a mold in the presence of a surface-treating agent comprising a contact angle-reducing substance under the condition that the contact angle between the liquid molding material and the inner surface of the mold is not more than 30xc2x0, and the liquid molding material is cured. When molded under this condition, there can be exhibited an excellent effect that a molded article having a shape accurately corresponding to the shape of the inner surface of a mold can be produced, without the generation of surface voids in the molded article. The reasons why this excellent effect is exhibited are not clear, but it is presumably based on the following. When the contact angle between the liquid molding material and the inner surface of the mold is reduced to not more than 30xc2x0, the wettability of the mold by the liquid molding material is improved, and thereby the friction between the liquid molding material and the mold is reduced. As a result, the liquid molding material can flow into the mold to follow the complicated shape of the inner surface of the mold. It is desired that the contact angle between the liquid molding material and the inner surface of the mold is not more than 23xc2x0, particularly not more than 21xc2x0, from the viewpoint of the reducing surface voids of the resulting molded article.
It is desired that the surface-treating agent comprises a contact angle-reducing substance, which reduces the contact angle between the liquid molding material and a flat plate made of the same material as the mold to not more than 30xc2x0 when the liquid molding material containing the surface-treating agent is dropped on the flat plate, or when the surface-treating agent is applied to the flat plate and the liquid molding material is dropped thereon. When this surface-treating agent is used, surface voids during molding can be reduced. More concretely, when the surface-treating agent is used, there can be exhibited an excellent effect that a molded article having a shape accurately corresponding to the shape of the inner surface of the mold can be easily produced without the generation of surface voids in the resulting molded article. The reasons why this excellent effect is exhibited are not clear, but it is presumably based on the following. The wettability of the mold by the liquid molding material is improved by the surface-treating agent, and thereby the friction between the liquid molding material and the mold is reduced. As a result, the liquid molding material can flow into the mold to follow the complicated shape of the inner surface of the mold. It is desired that the contact angle between the surface-treating agent and the inner surface of the mold is not more than 23xc2x0, particularly not more than 21xc2x0.
The contact angle can be determined in accordance with the following procedures.
1) The measurement atmosphere is kept windless at 25xc2x0 C. and 55% RH.
2) As the flat plate made of the same material as the mold, an aluminum plate [average roughness Ra: 0.2-0.4 xcexcm] is horizontally arranged. Thereafter, a silicone mold releasing agent is sprayed on its surface, and sufficiently wiped off with a waste cloth.
3) A given surface-treating agent is uniformly sprayed in the amount of 10 g/m2 on the silicone mold releasing agent-sprayed surface (surface-treating agents which are solid at 25xc2x0 C. are previously heated to a temperature not lower than their melting points to melt and then sprayed).
4) The liquid molding material previously degassed by evacuation is collected with a syringe, and one droplet is dropped on the flat plate from the height of 10 cm from the surface of the flat plate in a manner so that the droplet of the liquid molding material has the weight of 0.10xc2x10.02 g.
5) The time at which the droplet reaches the surface of the flat plate is counted as zero second. The change of the droplet is observed in accordance with the passage of time from the sideways direction of the droplet with a microscope as a CCD camera. After 60 seconds passed, the contact angle between the droplet and the flat plate is measured.
Examples of the contact angle-reducing substance include at least one compound selected from the group consisting of esters, ethers and amides, each having a boiling point of not less than 50xc2x0 C., preferably not less than 100xc2x0 C. at normal pressure, in particular a boiling point of not less than 100xc2x0 C. at normal pressure and a boiling point of not more than 300xc2x0 C. under the pressure of 0.133 kPa. Among the contact angle-reducing substances, those having vapor pressures of a level which does not completely evaporate within the working hours are preferable.
Examples of the ester having a boiling point of not less than 50xc2x0 C. include alkyl stearates wherein an alcohol residue moiety of the ester has 1 to 22 carbon atoms, such as ethyl stearate and butyl stearate; alkyl acetates wherein an alcohol residue moiety of the ester has 4 to 22 carbon atoms, such as decyl acetate and octadecyl acetate; alkyl oleates wherein an alcohol residue moiety of the ester has 1 to 22 carbon atoms, such as methyl oleate and butyl oleate; and other alkyl esters of fatty acids wherein the fatty acid residue moiety of the ester has an alkyl group of 2 to 21 carbon atoms, and the alcohol residue moiety of the ester has 1 to 22 carbon atoms, such as butyl propionate, butyl 2-ethylhexanoate, ethyl decanoate and methyl linoleate. Those esters can be used alone or in an admixture thereof. Among them, from the viewpoints of imparting an excellent effect of suppressing generation of surface voids to a resulting molded article and thus improving its appearance (design), alkyl esters of fatty acids, formed from a fatty acid and a monohydric alcohol, such as ethyl stearate, butyl stearate, decyl acetate, octadecyl acetate, methyl oleate and butyl acetate, can be suitably used. Further, it is desired that the fatty acid has 2 to 22 carbon atoms, preferably 2 to 18 carbon atoms, and that the monohydric alcohol has 1 to 22 carbon atoms, preferably 1 to 18 carbon atoms. Moreover, it is desired that the total number of carbon atoms of the fatty acid and the monohydric alcohol is 10 to 40, preferably 12 to 36. In addition, from the viewpoint of yellowing resistance of the resulting molded article, alkyl esters of saturated fatty acids, such as butyl stearate and octadecyl acetate, can be particularly suitably used.
The ester having a boiling point of not less than 50xc2x0 C. may be a symmetric ether or asymmetric ether. Examples thereof include dioctyl ether, dibutyl ether, dihexyl ether, didecyl ether, butyl hexyl ether, and the like. Those esters may be used alone or in an admixture thereof. Among them, from the viewpoints of imparting an excellent effect of suppressing generation of surface voids to a resulting molded article and thus improving its appearance (design), dioctyl ether, and the like can be suitably used.
The content of the contact angle-reducing substance in the surface-treating agent may be appropriately adjusted so that the contact angle between the liquid molding material and the flat plate is reduced to not more than 30xc2x0, preferably not more than 23xc2x0, more preferably not more than 21xc2x0. Incidentally, the contact angle-reducing substance itself can be solely used as the surface-treating agent, since the contact angle between the liquid molding material and the flat plate can be reduced to not more than 30xc2x0, even when the contact angle-reducing substance is used alone as the surface-treating agent.
The surface-treating agent may contain a mold releasing agent such as a silicone compound or a wax as occasion demands, in addition to the contact angle-reducing substance. The surface-treating agent containing the contact angle-reducing substance and the mold releasing agent can be desirably used by applying it to the inner surface of the mold during the production of various molded articles because this surface-treating agent suppresses the generation of surface voids, and improves the appearance (design) of the molded article.
The silicone compound can be suitably used in the present invention because the silicone compound is excellent in liquidity and releasing properties, and moreover is durable for repetitious use. Typical examples of the silicone compound include dimethyl silicone oil, and modified silicone oils such as alkyl-modified silicone oils and higher fatty acid-modified silicone oils, or those diluted with a solvent, those prepared into aqueous emulsions, and the like. Concretely, there can be cited dimethyl silicone oil having a viscosity of 30 to 5000 mm2/s at 25xc2x0 C. Those silicone compounds can be used alone or in an admixture thereof.
The wax can be suitably used because it is inexpensive. Examples of the wax include mineral oils, olefinic waxes, paraffin waxes, and the like. Those waxes can be used alone or in admixture thereof. Among them, the mineral oils and paraffin waxes each having a molecular weight of not less than 300 are preferable.
Among the mold releasing agents, the silicone compounds are more preferable from the viewpoint of the releasing property.
The content of the mold releasing agent in the surface-treating agent cannot be absolutely determined because it differs depending upon their kinds. However, it is desired that the content is usually not less than 5% by weight, preferably not less than 10% by weight, more preferably not less than 30% by weight, from the viewpoint of exhibiting sufficient releasing properties. Also, it is desired that the content is not more than 95% by weight, preferably not more than 90% by weight, from the viewpoint of exhibiting sufficient surface properties.
The surface-treating agent can be applied to the inner surface of the mold, or it can be contained in the liquid molding material.
When the surface-treating agent is applied to the inner surface of the mold, an outsole of shoe soles made of a polyurethane foam having complicated shapes particularly in the bottom surface can be formed in a shape exactly conforming to the mold without the generation of defects which impair the surface properties of the molded article, such as surface void.
The method for applying the surface-treating agent to the inner surface of the mold includes, for instance, a method of coating, spraying, dipping, or the like, without limiting the present invention to those methods above.
When the surface-treating agent is applied to the inner surface of the mold, the mold is filled with the liquid molding material thereafter, and can be molded under given molding conditions suitable for the kinds of the liquid molding material, and the like. It is desired that the amount of the surface-treating agent applied to the inner surface of the mold is 3 to 30 g/m2 from the viewpoints of imparting sufficient surface properties to a resulting molding article and preventing cracking and dulling of the surface.
When the surface-treating agent is contained in the liquid molding material, a midsole of shoe soles, made of a polyurethane foam and particularly having a complicated shape in the side surfaces can be molded in a shape exactly conforming to the mold without the generation of the defects which impair the surface properties of the molded article, such as surface void.
When the surface-treating agent is contained in the liquid molding material, the content of the surface-treating agent in the liquid molding material differs depending upon the kinds of the liquid molding material. However, it is desired that the content is adjusted so that the contact angle between the liquid molding material and the flat plate made of the same material as the mold is not more than 30xc2x0. For instance, it is desired that the content of the contact angle-reducing substance in the liquid molding material is 0.05 to 3.5% by weight, preferably 0.25 to 1% by weight.
When the surface-treating agent is contained in the liquid molding material, the liquid molding material is filled in the mold, and can be molded under given molding conditions suitable for the kinds of the liquid molding material.
The materials of the mold which can be used in the present invention are not particularly limited. Examples of such materials include iron, stainless steel, copper, aluminum, aluminum alloys, epoxy resins, phenolic resins, and the like. In addition, the shapes of the inner surface of the mold are not particularly limited, and any arbitrary shape can be selected as long as the inner surface has a shape well corresponding to the shape of the desired molded article.
When the liquid molding material is molded in a mold, it is desirable to previously apply a mold releasing agent to the inner surface of the mold by means of coating, spraying, dipping, or the like in order to improve its releasing property. The mold releasing agent includes dimethyl silicone oil, mineral oils, paraffin waxes, and the like, without intending to limit the present invention to those exemplified above.
Thus, a molded article having a given shape can be obtained by molding and subsequent demolding. The resulting molded article has excellent surface properties because the generation of harmful defects such as surface void is suppressed even when the molded article has a complicated shape in its inner surface.
Especially, when the molded article is a molded article made of a polyurethane foam, the above-mentioned effects are exhibited even more excellently. Particularly, the above effects are even more remarkably exhibited for the polyurethane foam for shoe soles having complicated shapes in the bottom surface or side surfaces.
In general, the shoe soles can be classified into outsoles used in sandals and men""s shoes, and midsoles used in sports shoes. The effects according to the present invention can be remarkably exhibited particularly in midsoles used within a low density region.
From the viewpoint of reducing surface voids, the density of the polyurethane foam is preferably not less than 0.15 g/cm3 and less than 0.30 g/cm3, and more preferably not less than 0.20 g/cm3 and less than 0.30 g/cm2.
In a mixer (Model xe2x80x9cDH-2.5,xe2x80x9d manufactured by Tokushu Kika Kogyo K.K.) were previously stirred 50 parts by weight of a polyol solution [trade name: xe2x80x9cEDDYFOAM AS-2045,xe2x80x9d manufactured by Kao Corporation], comprising polypropylene glycol, a chain extender, water and a foam stabilizer (a surfactant), and 50 parts by weight of an isocyanate prepolymer [trade name: xe2x80x9cEDDYFOAM B-6009N,xe2x80x9d manufactured by Kao Corporation], main constituents of which were polypropylene glycol and 4,4-diphenylmethane diisocyanate, and the resulting mixture was evacuated in a desiccator.
The resulting liquid mixture was used to measure a contact angle in accordance with the following method for measurement of a contact angle. As a result, the contact angle was found to be 43xc2x0.
The viscosity of this liquid mixture gradually increases. Therefore, the time period from the previous stirring of the polyol solution and the isocyanate prepolymer to the measurement of the contact angle was adjusted to four minutes.
In a mixer (Model xe2x80x9cDH-2.5,xe2x80x9d manufactured by Tokushu Kika Kogyo K.K.) were previously stirred 50 parts by weight of a polyol solution [trade name: xe2x80x9cEDDYFOAM AS-1210U,xe2x80x9d manufactured by Kao Corporation], comprising a polyester-polyol, a chain extender, water and a foam stabilizer (a surfactant), and 50 parts by weight of an isocyanate prepolymer [trade name: xe2x80x9cEDDYFOAM B-2009,xe2x80x9d manufactured by Kao Corporation], main constituents of which were a polyester-polyol and 4,4-diphenylmethane diisocyanate, and the resulting mixture was evacuated in a desiccator.
The resulting liquid mixture was used to measure a contact angle in accordance with the following method for measurement of a contact angle. As a result, the contact angle was found to be 46xc2x0.
The viscosity of this liquid mixture gradually increases. Therefore, the time period from the previous stirring of the polyol solution and the isocyanate prepolymer to the measurement of the contact angle was adjusted to four minutes.
[Measurement of Contact Angle]
1) The measurement atmosphere is kept windless at 25xc2x0 C. and 55% RH.
2) As the flat plate made of the same material as the mold, an aluminum plate [average roughness Ra: 0.2-0.4 xcexcm] is horizontally arranged. Thereafter, a silicone mold releasing agent [trade name: xe2x80x9cPURAPOWER 2060,xe2x80x9d manufactured by Kao Corporation] is sprayed on its surface, and sufficiently wiped off with a waste cloth.
3) A given surface-treating agent is uniformly sprayed in the amount of 10 g/m2 on the silicone mold releasing agent-sprayed surface (surface-treating agents which are solid at 25xc2x0 C. are previously heated to a temperature of not lower than their melting points to melt and then sprayed).
4) The liquid molding material previously degassed by evacuation is collected with a syringe [Model xe2x80x9cSS-02S,xe2x80x9d manufactured by TERUMO CORPORATION], and one droplet is dropped on the flat plate from the height of 10 cm from the surface of the flat plate in a manner so that the droplet of the liquid molding material has the weight of 0.10xc2x10.02 g.
5) The time at which the droplet reaches the surface of the flat plate is counted as zero second. The change of the droplet is observed in accordance with the passage of time from the sideways direction of the droplet with a microscope [Product No. xe2x80x9cVH-6200,xe2x80x9d manufactured by KEYENCE CORPORATION] as a CCD camera. After 60 seconds passed, the contact angle between the droplet and the flat plate is measured.